Weapon stabilization systems are used on tanks to stabilize the position of the gun tube so that the line of fire can be controlled while the tank is in motion. The operator views the surrounding terrain through a sight head mirror located in the turret. Rotation of the turret permits the operator to look to the left or right of the current view. The sight head mirror is pivotable about a horizontal axis so that, within certain limits, the operator can look above or below the current view. The sight head mirror has its own, independent stabilization system to maintain its position such that it maintains the view selected by the operator regardless of terrain induced disturbances. For targeting purposes a reticle is superimposed on the view given by the sight head mirror. Targeting of an object is then simply accomplished by activating the operator controls to align the reticle with the object. Activation of the operator controls also causes the gun tube to move by an amount which corresponds to that of the sight head mirror. Since the gun tube is aligned in conjunction with the sight head mirror, the line of fire would intersect the object being targeted, except for the existence of a ballistic solution which corrects for speed of the targeted object (in the case of a moving target) and for elevational requirements due to the trajectory of the bullet.
The goal of the weapon stabilization system is therefore to maintain the line of fire in the direction selected via the operator controls, regardless of terrain induced disturbances or other influencing factors. This is accomplished in conventional stabilization systems by assuming that the position of the gun tube, as measured at the gun mount, gives the exact position of the gun muzzle (i.e., the discharging end of the gun tube).
However, in the quest for longer ranging direct fire weapons, tank gun tubes are becoming longer, bringing with them associated stabilization problems. For example, longer gun tubes tend to bend or flex to a degree sufficient to adversely affect targeting accuracy. This flexion can be the result of many factors, such as differential thermal warming or cooling (thermal bending), vertical or heave acceleration, and firing of the gun. The result of this bending of the gun tube is deflection of the muzzle from its desired position. Therefore, the assumption of conventional stabilization systems that the muzzle position (and, thus, the line of fire) can accurately be determined by monitoring the position of the gun tube at the gun mount does not hold true for longer, flexible gun tubes.
It is known in the prior art to adjust the position of the gun tube to account for thermal bending by a system which utilizes a muzzle reference sensor having a transmitter/receiver located at the breech of the gun to reflect and sense light off a mirror located at the gun muzzle. This muzzle reference sensor operates or samples at approximately 60 Hz; fast enough to account for thermal effects, which have time constants on the order of minutes, but not fast enough to account for higher frequency effects, such as terrain induced disturbances and gun firing reaction. More recently, a continuous muzzle reference sensor has been developed which has sufficient bandwidth to measure these higher frequency flexions. However, no one has heretofore provided a system for controlling these deflections of the muzzle. Rather, weapon stabilization systems continue to operate on the erroneous assumption that the muzzle position is accurately determinable by measuring gun tube position at the gun mount. It would therefore be desirable to have a weapon stabilization system that reduces the error in muzzle position caused by flexion of the gun tube.
Another problem that arises with the use of longer gun tubes is that, whereas the center of gravity of the gun tube has traditionally been designed to coincide with the trunnion axis, the longer tubes, in conjunction with other constraints such as weight and space, have resulted in the center of gravity being offset from the trunnion axis in a direction toward the muzzle. The gun tube is therefore unbalanced at its pivot point. The gun tube will thus experience translational and rotational accelerations due to disturbances caused by the terrain. As used herein, these accelerations are referred to as external accelerations because they are accelerations of the gun tube that are not caused by operation of the actuator.
These external accelerations provide a torque that backdrives the actuator that controls the elevational position of the gun tube. Often, the actuators used are hydraulic actuators and the stabilization system includes pressure feedback from the actuator in the form of negative feedback that dampens the response of the actuator to the command sent from the operator controls. In such systems, the externally applied torque due to the imbalance of the gun tube creates undesirable positive feedback to the actuator that moves or tends to move the actuator in the direction of the backdriving torque. Thus, for example, an external force directed downward at the gun muzzle creates feedback to the actuator that tends the move the muzzle downward. This result is undesirable because the stabilization system should maintain the chosen line of fire irrespective of external forces on the gun tube.
The external accelerations acting on the actuator due to the imbalance of the gun tube can be categorized as either static or dynamic. Static, or one-g external acceleration is that due to the effect of earth's gravity. Dynamic external acceleration is that due to other external accelerations, such as terrain induced disturbances. For example, if the tank hits a bump while moving it may experience two-g's of acceleration, the static one-g plus one-g due to the upward movement of the tank as a result of encountering the bump in the terrain.
It is known to provide a separate stabilization system to account for unbalance due to static acceleration of the gun tube. One such system includes a vessel of pressurized nitrogen gas coupled into the actuator to bias the actuator by an amount equal and opposite to the static force due to the imbalance of the gun. Mechanical arrangements have also been described, as exemplified by U.S. Pat. Nos.: 5,014,594, issued May 14, 1991 to Muhlhausen et al.; 5,101,708, issued Apr. 7, 1992 to Sommer et al.; and 5,196,642, issued Mar. 23, 1993 to Tripp. Muhlhausen et al. and Sommer et al. utilize a torsion bar suspension mechanism to counteract the unbalance. Tripp utilizes a wire cable extending about a contoured cam surface with one end connected to the weapon barrel and the other end connected to a pneumatic cylinder that operates to extend or retract the cable. The compensating force is provided by the pneumatic cylinder, with a magnitude determined by the contour of the cam surface.
These unbalance compensation systems are disadvantageous primarily because they do not counteract for the dynamic torques that a tank or other movable platform is likely to encounter. It would therefore be desirable to have an unbalanced weapon stabilization system utilizing a hydraulic actuator with pressure feedback that accounts for the positive feedback created due to dynamic external accelerations.